Polyester fibers are used in various fields typified by clothing applications because of their properties such as mechanical properties, color development, and handling. However, polyester fibers are generally inferior in dyeability because of their dense fiber structure and when cation dyes and disperse dyes are used, it is difficult to obtain good color development and fastness property unless such fibers are put under high temperature of 130° C. and high pressure or a carrier of an organic solvent is used.
On the other hand, there has been demanded a technology for producing a polyester mixed product having good dyeing properties by mix-knitting or mix-weaving a polyester fiber with a material other than polyester, such as wool, cotton, acrylics and polyurethane, without using complicated steps; in this case, in order to provide sufficient dyeing properties to a polyester fiber, dyeing processing under high temperature of about 130° C. and high pressure is needed. However, since the material mix-knitted or mix-woven with the polyester fiber is degraded under the environment, there have been required development of a polyester fiber having good dyeing properties even under, for example, a normal pressure environment, more specifically, under 100° C. or lower.
For this reason, methods of improving dyeability through modification of a polyester resin have been studied in large numbers. Above all, there are many proposals to produce a polyester fiber easily dyeable with cation dyes and disperse dyes under normal pressure by copolymerizing a metal sulfonate group as a dicarboxylic acid component (see, for example, Patent Documents 1 to 4). Substances commonly used as a dicarboxylic acid having a metal sulfonate group include a 5-sodium sulfoisophthalic acid component, a 5-potassium sulfoisophthalic acid component, and the like. For example, Patent Documents 2 and 4 disclose a polyester fiber in which a 5-sodium sulfoisophthalic acid component and an adipic acid component are copolymerized. It has been reported that by processing such a copolymerized polyester into a fiber, it allows a fiber internal structure to hold amorphous parts well in comparison with conventional polyester fibers. And, it has been reported that, as a result, a polyester fiber can be obtained which is capable of being normal pressure dyed with disperse dyes and cation dyes and superior in fastness. However, fibers prepared by copolymerizing dicarboxylic acids having a metal sulfonate group are insufficient in dyeability under normal pressure.
Moreover, Patent Document 5 discloses a polyester-based conjugated fiber including a polyester A, as a core component, containing ethylene terephthalate units as main repeating units and a polyester B, as a sheath component, containing ethylene terephthalate units as main repeating units and obtained by being copolymerized with 5 to 30 mol % of cyclohexanedicarboxylic acid, wherein the glass transition temperature of the polyester B is lower than the glass transition temperature of the polyester A. Polyester-based conjugated fibers obtained in such a way are reported to be superior in dyeability and deep color property. However, the dyeability under a normal pressure environment may be insufficient in some cases and therefore improvement has been desired.
Moreover, fiber structures, such as woven fabric, knitted fabric and nonwoven fabric, made of a synthetic fiber, for example, a filament of polyester or polyamide are heretofore colder and more monotonous in feeling and gloss as compared with natural fibers such as cotton and hemp and therefore are low in quality as fiber structure because their monofilament fineness or cross-sectional shape of the constituent filament is monotonous. In addition, since polyester fibers are hydrophobic, there is a defect of being inferior in the water absorptivity and hygroscopicity of the fibers themselves. In order to improve these defects, various studies have been made. Out of them, there has been made, for example, an attempt to impart such performance as hydrophilicity to a hydrophobic fiber by conjugately spinning a hydrophobic polymer such as polyester and a polymer having a hydroxy group. Specifically, conjugated fibers made of an ethylene-vinyl alcohol-based copolymer and a hydrophobic thermoplastic resin, such as polyester, polyolefin, and polyamide, are disclosed (see, for example, Patent Documents 6 and 7).
Conjugated fibers made of an ethylene-vinyl alcohol-based copolymer and a polyester have a defect that the constituent polymers are prone to peeling because of low adhesion at the interface between the polymers. In order to improve this defect, various studies have been made. Out of them, there is disclosed a conjugated fiber superior in the interfacial peeling resistance in which projections are formed at the interface between the conjugated components as observed in the photograph of a cross-section of the fiber of FIG. 1 or 3 (see, for example, Patent Document 8).
However, conjugated fibers made of an ethylene-vinyl alcohol-based copolymer and a polyester have a defect that they are poor in heat stability against high temperature hot water and steam because of the low melting point or softening point of the ethylene-vinyl alcohol-based copolymer. For this reason, the conjugated fiber allows the ethylene-vinyl alcohol-based copolymer exposed on the surface of a fiber product, such as a woven fabric, a knitted fabric, and a non-woven fabric, to partly soften or slightly conglutinate by high-temperature, high-pressure dyeing or the use of a steam iron, so that the feeling becomes hard as a fiber product. In order to prevent this, there has been disclosed a method in which hydroxyl groups of the copolymer are acetalized using a dialdehyde compound or the like before bringing the fiber into contact with high-temperature hot water during dyeing or the like. However, the acetalization treatment causes a problem of increase in processing cost because it needs since this acetalization treatment needs an acetalization step in addition to the current dyeing step. Moreover, a problem regarding the corrosion resistance of a treating apparatus also arises due to the use of a strong acid in a high concentration in the acetalization treatment. Furthermore, a problem of difficulty in increasing color depth also arises because dyes are less prone to diffuse into the inside of the fiber acetalized. In addition, there also arises a problem of discoloration or the like of a dyed material caused by unreacted dialdehyde compounds in the acetalization treatment. Therefore, there were problems with the securement of the uniformity of fiber performance. Moreover, it is difficult to determine what type of dialdehyde compound and what degree of acetalization to be used for industrial practice of acetalization treatment depending upon the type of the compound and the degree of acetalization for the treatment. Therefore, it was a technology lacking stability for practical use. In other words, color difference occurs in dyed materials depending upon the degree of crosslinking, so that stable feeling failed to be obtained and only products with very low commercial value were obtained.